The invention concerns a compact and reliable singly-resonant optical parametric oscillator (SRO) capable of emitting laser light of high spectral purity and frequency stability over a wide spectral range.
An optical parametric oscillator (OPO) is a nonlinear device which converts incident photons into photon pairs when optically excited at a power per unit area above a certain threshold. The threshold level is a characteristic of the non-linear material, the resonator, and is a function of wavelength. This device is usually embodied in one of two forms: Either a doubly-resonant oscillator (DRO) in which both the generated optical beams are resonated or in a singly-resonant oscillator mode (SRO) in which only one of the generated optical beams is in resonance.
Use of optical parametric oscillators for commercial and scientific applications requires simultaneous achievement of several requirements. In particular, widely tunable laser radiation having high frequency stability and narrow linewidth is usable for a plurality of applications in the field of high-resolution spectroscopy and metrology. Continuous-wave operation of such laser sources is required to achieve linewidths on the order of one Mega Hertz or less. A plurality of continuous-wave lasers are available for different portions of the optical spectrum e.g. laser diodes in the 630-2000 nanometer range, titanium-sapphire lasers in the 710-1100 nanometer range, dye lasers in the 400-800 nm range and color center lasers in the 2-3.5 .mu.m spectral regions. However, these lasers fail to simultaneously satisfy the following criteria:
Large emission range (in excess of 100 nm); PA1 High power (in excess of 50 mW); PA1 Narrow linewidth (less than 1 Mega Hertz); PA1 Good frequency stability (drift less than 200 MHz/h); and compact size.
In principle, nonlinear optical frequency conversion can be used to extend the wavelength range of lasers having the desired properties. In combination with solid-state lasers, such as diode-pumped Nd:YAG lasers, pulsed nonlinear frequency conversion has been demonstrated to be capable of generating light in the ultraviolet, visible and infrared spectral regions in compact, powerful, and reliable systems. Research on continuous-wave optical parametric oscillators (OPOs) driven by diode-pumped solid state lasers had been started in 1989 by Kozlovsky et al. (Optics Letters 14, 66 (1989)) using a doubly-resonant OPO (DRO) with both generated waves being resonantly enhanced to reduce the oscillator threshold. Although emission ranges of more than 200 nm in the near infrared region and output powers in the mW range had been demonstrated (Gerstenberger et al., J. Opt. Soc. Am. B 10, 1681 (1993)), the high susceptibility of DROs to mode-hopping and the difficult tuning behavior (Eckardt et al., J. Opt. Soc. Am. B 8, 646 (1991)) have caused continuous-wave OPOs to achieve the reputation of being non-suitable for high-resolution spectroscopy applications. Yang et al. (Optics Letters 18, 971 (1993)) have shown that a singly-resonant OPO (SRO) can achieve mode-hop-free operation over several minutes and a continuous tuning range of 550 MHz has been obtained. These achievements of prior art are still far from the practical demands of high-resolution spectroscopy applications.
In view of these disadvantages of prior art, it is the principal purpose of the present invention to further improve a singly-resonant oscillator of the above mentioned kind in such a fashion that frequency-stable and mode-hop-free operation with continuous frequency tuning is achieved in an efficient, compact, stable and widely tunable nonlinear frequency conversion system.